Gonadal steroids are essential for the expression of normal male mating behavior. Three areas of the limbic system, the medial nucleus of the amygdala (M), the be nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA), are targets for steroid actions on male mating behavior. Neurons in these ares contain neuroactive peptides, and the levels of these peptides are regulated by gonadal steroids. Thus, gonadal steroids may regulate mating behavior by regulating levels of neuropeptides. To further test this hypotheses it is important to 1) identify the neuropeptides and determine whether specific neuropeptides are present in separate neurons, or whether they co-occur in individual neurons; 2) identify the location of sources of afferents, and the efferent targets of these peptide specific populations; and to 3) determine the role of neuropeptides in M, MPOA and BNST in the regulation of male mating behavior. Moreover, to clarify the cellular mechanisms by which steroids regulate peptide levels, it is important to 1) determine whether testosterone, or one of its metabolites, regulates peptide levels in these neurons, and 2) determine whether steroids act directly on peptidergic neurons. The proposed experiments will provide this information using anatomical and behavioral techniques. These studies will clarify both the role of neuropeptides in the regulation of male mating behavior, and the cellular mechanisms by which gonadal steroids influence the function of neurons involved in the regulation of mating behavior.